In the design of modern transportation vehicles, structural vibration and interior noise have become important problem areas that must be addressed. For example, in helicopter systems, control of sound transmission into enclosed spaces is an important issue. Various studies have shown that the predominant frequency components associated with the noise transmission lie in the frequency range of 50 Hz to 5500 Hz. There are various approaches that may be used to minimize sound within a helicopter cabin.
One approach, which is based on controlling the radiation (transmission) from (through) a flexible structure by active means, is referred to as Active Structural Acoustic Control (ASAC). The ASAC scheme, which is an effective solution for low frequency applications, takes advantage of vibrating structural elements as secondary noise sources to cancel the sound fields generated by a primary noise source (A. Sampath, et al., “Active Control of Multiple Tones Transmitted in an Enclosure”, Journal of the Acoustical Society of America, Vol. 106, No. 1, Pages 211-225, July 1999; M. Al-Bassyiouni, et al., “Zero Spillover Control of Enclosed Sound Fields”, SPIE's Annual International Symposium of Smart Structures and Materials, Newport Beach, Calif., March 4-8, Vol. 4362, Paper No. 4326-7, 2001; and, M. Al-Bassyiouni, et al., “Experimental Studies of Zero Spillover Scheme for Active Structural Acoustic Control Systems”, Proceedings of the 12th International Conference on Adaptive Structures and Technologies (ICAST), University of Maryland, College Park, Md., Oct. 15-17, 2001). It appears that ASAC schemes require much less dimensionality than Active Noise Control (ANC) schemes in order to realize widely distributed spatial noise reduction. As known in the art, ANC schemes are generally used to minimize noise by using various cancellation techniques. However, active research is still being pursued to address issues such as sensors, actuators, and control architecture.
Fiber-optic sensors have the advantages of being lightweight, having high sensitivity, and provide simplicity in multiplexing. Demonstrations have showed that optical fibers may be used as acoustic sensors (Bucaro J. A., et al., “Fiber Optic Hydrophone”, Journal of Acoustical Society of America, 62, Pages 1302-1304, 1977; and, Cole, J. H., et al., “Fiber Optic Detection of Sound”, Journal of Acoustic Society of America, 62, Pages 1136-1138, 1977). Much of the research in this area has been directed towards the development of hydrophones for ultrasonic detection which does not suit the needs of an ASAC system.
Since Bragg grating sensors were shown to be multiplexible by using Wavelength Division Multiplexing (WDM) techniques, Baldwin, et al., (“Bragg Grating Based Fabry-Perot Sensor System for Acoustic Measurements”, Proceedings of the SPIE 1999 Symposium on Smart Structures and Materials, Newport Beach, Calif., Mar. 1-5, 1999), developed a Bragg grating based Fabry-Perot sensor system for use in ASAC schemes. However, the sensor bandwidth was found to be limited, and in addition, the sensor was found to have low sensitivity due to the high Young's modules of silica resulting in “acoustically induced strains” which also limit the application of this type of sensors.
Thus, low finesse Fabry-Perot sensors have become attractive choices for high performance sensing in this area. As shown in the prior art, a Fabry-Perot optical sensing device for measuring a physical parameter, described in U.S. Pat. No. 5,392,117 comprises a Fabry-Perot interferometer through which a multiple frequency light signal having predetermined spectral characteristics is passed. The system further includes an optical focusing device for focusing at least a portion of the light signal going outwards from the Fabry-Perot interferometer and a Fizeau interferometer through which the focused light signal is passed.
The Fabry-Perot interferometer includes a pair of semi-reflecting mirrors substantially parallel to one another and spaced apart so as to define a Fabry-Perot cavity having transmittance or reflectance properties that are effected by a physical parameter such as pressure, temperature, refractive index of a liquid, etc., which causes the spectral properties of the light signal to vary in response to changes in physical parameters.
The Fabry-Perot interferometer is provided with at least one optical fiber for transmitting the light signal into the Fabry-Perot cavity for collecting the portion of the light signal being transmitted outwards. The Fizeau interferometer includes an optical wedge forming a wedge-profile Fizeau cavity from which exits a spatially-spread light signal indicative of the transmittance or reflectance properties of the Fabry-Perot interferometer.
Of particular interest are sensor configurations that may be used for various acoustic measurements, such as measurement of sound pressure gradients, air particle velocity, and acoustic intensity. Currently, there are no commercially available fiber optic sensor systems which may be used for these measurements since the current technology is primarily based on condenser microphones.
Velocity sensors have numerous advantages, some of which are as follows: (1) better sensitivity to spherical waves compared to the sensitivity of a pressure microphone; (2) can be used along with the pressure microphones to measure the sound energy density; and (3) can be used along with pressure microphones to develop a unidirectional microphone that would favor waves incident from only one direction and discriminate waves incident from other directions.
The concept of a typical velocity microphone is known in the prior art. However, complexity and bulkiness of known velocity microphones makes them difficult to use effectively in ASAC systems. A conventional arrangement of a velocity microphone consists of a corrugated metallic ribbon suspended between the N and S magnetic pole pieces and freely acceptable to acoustic pressures on both sides (L. E. Kinsler, et al., “Fundamentals of Acoustics”, Second Edition, John Wiley & Sons, Inc., New York, 1962).
The ribbon acts as a short light cylinder that may be easily displaced in one direction under a force generated by air pressure. A velocity sensor was proposed (J. W. Parkins, “Active Minimization of Energy Density in a Three-Dimensional Enclosure”, Ph.D. Dissertation, Pennsylvania State University, 1998) which consists of six pressure condenser microphones mounted on a sphere of radius of 1.0 inch.
A finite difference scheme was used to predict the air particle velocity from the pressure measurement. Although the size of the sensor was “small” compared to many commercially available velocity probes, it was shown that such a sensor could lead to errors if there is any mismatching between the different pressure microphones.
There may also be the potential for interference, since a plurality of microphones are generally housed together in a small volume. This interference may significantly affect the sensor signal-to-noise ratio, especially at low sound pressure levels. It is thus clear that a velocity sensor free of the disadvantages of prior art velocity sensors is needed in industry.
A new technology has been introduced recently by Microflown, a Dutch company, which allows for small scale air particle velocity sensors. However, these sensors are dependent on thermal effects, and therefore, operate at very high temperatures.
Summarizing the discussion of the prior art supra, it is readily understood to those skilled in the art that there is needed a wide bandwidth (in the frequency range of 50 Hz to 7.5 KHz) fiber tip based Fabry-Perot sensor systems for (acoustic) pressure measurements, which is free of the disadvantages of the prior art acoustical measurement systems, and which is capable of serving as a pressure gradient sensor, a velocity sensor, and an acoustic intensity sensor, and further is electrically passive and considerably smaller in size than the sensor systems based on condenser microphones.